This invention relates to self-contained valves and, more particularly, to self-contained, dynamically-stopped and statically balanced valves.
A valve that passes steam, water or some other fluid medium and uses that same medium to generate most of the force necessary to operate the valve is known as a self-contained or pilot-operated valve. For example, as shown in the Fulton U.S. Pat. No. 1,176,535, a control fluid enters an inlet opening, moves through a valve seat opening and then exits via an outlet opening. A spring positioned about a valve plug stem urges a valve plug into a valve seat. Located about the valve stem is a metal bellows attached by a plate to the valve stem.
Provision is made to permit the fluid from the inlet opening to enter the bellows and support the closing force of the spring. By means of a pilot valve, the pressure within the bellows can be reduced rapidly. As a result, the pressure outside the bellows exceeds the pressure within the bellows and the bellows begins to collapse. This has the effect of overcoming the spring and lifting the valve plug from its seat. A complete collapse of the bellows in its axial direction is prevented by means of a mechanical stop.
When the valve is to be closed, the pilot valve is operated to close the relief line from the interior of the bellows. The pressure within the bellows then begins to increase until it matches the pressure outside the bellows. When this occurs, the spring can then return the valve plug to its seat, closing off the flow of fluid.
Other patents disclosing self-contained or pilot-operated valves include the Papulski U.S. Pat. No. 2,291,101, the Fulton U.S. Pat. No. 2,590,853 and the Scaglione U.S. Pat. No. 3,493,008.
A standard technique in the design of self-contained valves is the use of a balancing piston to reduce the force necessary to lift the valve plug away from its seat. The use of such a balancing piston is generally effective to balance out the forces over most of the stroke provided that a small sealing annulus is used. A drawback with such arrangements is that imbalance forces are created as the valve plug leaves its seat. These forces make controlling the valve at the opening area extremely difficult, especially when soft-seated valves are employed. One way of dealing with this problem is to use a high rate spring, with a resulting large control area.